Airstrikes carried out by Syrian government or Russian jets hammered rebel-held areas of the northwestern Idlib province on Tuesday leaving dozens of civilians dead, according to activists and medical workers who say chemicals weapons were likely used.

Horrific video and photos posted online by the Syrian Civil Defense, the volunteer first-responders organization commonly known as the White Helmets, showed young children who had purportedly died in the attack on the town of Khan Sheikhoun.

A member of the White Helmets team in Idlib posted a message on Twitter saying at least 37 civilians were killed and hundreds injured in what he called a “poisonous gas attack” on the town. The images he posted showed children -- at least eight in one video -- seemingly deceased without any apparent external injuries.

The Syrian Observatory for Human Rights (SOHR), a U.K.-based activist group that relies on a network of contacts inside Syria and which generally proves an accurate source of information, said at least 58 people were killed, including 11 children. SOHR said the strikes left many victims chocking for air, and cited local medical workers as describing the effects of a poison gas attack.

Later Tuesday, the Idlib Health Directorate gave a death toll of 100, with hundreds more injured. An umbrella opposition group said 70 were dead. The varying death tolls could not be independently reconciled as the area is rebel-held and it is not safe for journalists to travel there.

Hours after the alleged chemical attack, the White Helmets said a field hospital in the same area was hit by a rocket, destroying the facility and damaging vehicles. It wasn’t immediately clear whether there were additional casualties from the purported strike.

Syrian President Bashar Assad has been accused many times during the country’s six-year civil war of using illegal, indiscriminate weapons against his own people, including chemical weapons. He has consistently denied the accusations, in spite of overwhelming evidence.

A doctor working in Idlib province, identifying himself on Twitter as a British-trained physician currently volunteering as a “humanitarian aid worker in northern Syria,” posed a video in which he demonstrates how survivors of the alleged Khan Sheikhoun attack have “pin-point pupils that do not respond to light.” He claims it is evidence that the strikes used the banned chemical weapon Sarin gas.

Several United Nations Security Council Resolutions aimed at punishing Syria for using the banned weapons have been blocked by Assad’s most valuable ally, Russia, and by China.

Hamish de Bretton Gordon, a former chemical and biological weapons chief for the British army, told CBS News that, judging from the images and video posted online from Khan Sheikhoun, the attack seemed more likely to have involved a nerve agent than the more readily available and far less lethal chlorine.

“It does appear to be a nerve agent. If the casualty toll is as high as they claim then it’s likely Sarin,” de Bretton Gordon said. “Even the biggest chlorine attacks have only seen one or two deaths with 30 plus injuries.”

He said it was unlikely a Mustard gas attack as the victims showed no external blistering.

The assessment was shared by another chemical weapons expert who told CBS News on Tuesday that the videos appeared to show victims of a possible Sarin gas attack.

“It is looking distinctly like an attack with something considerably more potent than chlorine or mustard gas. A nerve agent like Sarin would fit the descriptions I have read of symptoms,” Alastair Hay a professor of environmental toxicology at Leeds University in England, said.

De Bretton Gordon said the images reminded him of the devastating 2013 attacks by Assad’s forces on the eastern Ghouta suburbs of Damascus, which left many hundreds dead and almost led to U.S. military action against Damascus. International inspectors concluded it was a Sarin gas attack.

While the Ghouta attack clearly crossed then-President Obama’s “red-line” for the Assad regime, a deal was negotiated with Russia that was meant to see Assad hand over all of his chemical weapons stockpile for destruction. That deal averted U.S. military action.

As the death toll in Khan Sheikhoun continued to rise on Tuesday, Assad’s government reiterated it’s long-standing, generic denial of chemical weapons use, saying the Syrian military “does not and has not” used them.

A high-ranking Syrian military officer told CBS News, on condition of anonymity, that the “allegations are null and void. Syrian or Russian jets never used chemical weapons during their fight against terrorism.” He said Syrian forces and their allies “target terrorist groups, not civilians.”

The Organization for the Prohibition of Chemical Weapons, the global chemical weapons watchdog that facilitated the destruction of Assad’s known chemical weapons stockpiles four years ago, said Tuesday that it was “in the process of gathering and analyzing information from all available sources” related to the Khan Sheikhoun attack.

Several United Nations Security Council Resolutions aimed at punishing Syria for using the banned weapons have been blocked by Assad’s most valuable ally, Russia, and by China.

Hamish de Bretton Gordon, a former chemical and biological weapons chief for the British army, told CBS News that, judging from the images and video posted online from Khan Sheikhoun, the attack seemed more likely to have involved a nerve agent than the more readily available and far less lethal chlorine.

“It does appear to be a nerve agent. If the casualty toll is as high as they claim then it’s likely Sarin,” de Bretton Gordon said. “Even the biggest chlorine attacks have only seen one or two deaths with 30 plus injuries.”

He said it was unlikely a Mustard gas attack as the victims showed no external blistering.

The assessment was shared by another chemical weapons expert who told CBS News on Tuesday that the videos appeared to show victims of a possible Sarin gas attack.

“It is looking distinctly like an attack with something considerably more potent than chlorine or mustard gas. A nerve agent like Sarin would fit the descriptions I have read of symptoms,” Alastair Hay a professor of environmental toxicology at Leeds University in England, said.